Presently, high performance brake disks for aircraft are frequently made by densifying carbon preforms through chemical vapor deposition using a gaseous methane precursor to form a carbon/carbon composite. This densification process typically involves repeated cycles consisting of densification followed by machining of the preform surface to open pores closed during densification. One disadvantage of this technique is that it can require large amounts of time, sometimes on the order of hundreds of hours to fully densify the preform.
Alternatively, the densification must be performed slowly so that the pores on the outside of the preform do not get filled before the pores on the inside of the preform. Were the pores on the outside of the preform to be blocked before densification of the interior portions, insufficient precursor would reach the inner portions of the preform and it would not be fully densified.
One approach to avoid this problem is by using a liquid hydrocarbon precursor densification process, such as is disclosed in U.S. Pat. No. 5,389,152, issued on Feb. 14, 1995 to Thurston et al. In this process, overall densification occurs from the inside of the preform outward. Densification can, thus, be performed at a higher rate without concern that the pores at the exterior of the preform will be blocked and prevent densification of the preform interior.
During densification with a liquid precursor, the temperature gradient is such that the center of the preform is hotter than the surface and thus the densification proceeds from the center to portions of the surface as the temperature of the preform is increased. Typically, portions of the preform near some of the surfaces do not densify using this process. As a result, portions of the edges of the preform are often machined away after densification using a liquid precursor to form a fully densified structure. However, this process increases the waste associated with densification.
Therefore, a need exists for an improved method that allows densification of the regions of a preform that do not normally densify when using a liquid precursor.